This invention relates to a wound golf ball which exhibits a good spin performance upon different club shots and affords satisfactory feel, durability, and distance.
Wound golf balls include balls of the type in which a wound core obtained by winding thread rubber about a solid center is enclosed with a cover of an ionomer resin or balata rubber. For improving wound golf balls of this type, there were proposed several approaches including increasing the diameter and reducing the hardness of the solid center. More particularly, (1) the approach of increasing the diameter and reducing the hardness of the solid center is disclosed in JP-A 59-129072, 60-168471, 60-72573, 5-337217, 6-54930, 7-313630, 8-224323, 9-271537, and 10-108922; and (2) the approach of narrowing the hardness distribution of the solid center is disclosed in JP-A 6-238013, 9-271539, and 10-201881.
These proposals, however, have the following problems.
(1) Problems associated with large-diameter and low-hardness solid centers:
(i) By increasing the diameter and reducing the hardness of the solid center, the spin rate of the ball when hit is reduced, which is effective for increasing the distance. However, since the spin rate is reduced on all shots, the ball upon approach shots does not stop on the green as desired, leading to difficult ball control.
(ii) The solid center tends to deform upon thread winding, making it difficult to ensure the quality.
(iii) The thread rubber layer must be accordingly reduced, making it difficult to provide resilience.
(2) Problems associated with narrow hardness distribution solid centers:
If the hardness distribution from a middle to the surface of a solid center is narrow, the solid center in its entirety deforms upon every shot from a driver to a short-iron, to an extent depending on a particular type of club. If the hardness of the solid center is made uniformly low (soft) so as to reduce the spin rate upon driver shots for increasing the distance, the spin rate upon short-iron shots is also reduced, leading to a shortfall of controllability. Inversely, if the hardness of the solid center is made high (hard) for improving controllability, the spin rate upon driver shots is also increased, leading to a shortfall of distance.
Where the head speed is low enough to produce less deformation of the golf ball as in the case of approach shots, the cover makes a more contribution to the spin rate than the deformation of the ball. For increasing the spin rate for better stop on the green, the cover must then be made soft. When such a soft cover is combined with the improved solid center mentioned above, strikes with other clubs also produce increased spin rates. Then in head wind, the ball will sky high and travel short. It is difficult to provide the ball with a spin performance complying with all types of clubs including a driver and short irons.
An object of the invention is to provide a wound golf ball having a spin performance complying with all types of clubs and affording a good feel, durability, and increased distance.
It has been found that the spin performance of a golf ball is determined by the deformation of the ball and the friction between the ball and the club face upon impact. The spin rate decreases as the deformation of the ball upon impact becomes greater. Also the spin rate decreases as the cover becomes harder and undergoes less deformation.
To improve the controllability of the ball upon iron shots, it is necessary to make the cover soft so as to receive more spin. However, a mere attempt to soften the cover for increasing the spin rate produces a ball which will travel a too high trajectory when hit with a driver or iron against head wind, covering a shorter distance. This necessitates a study to optimize the deformation of the ball relative to the cover hardness.
The invention is directed to a wound golf ball comprising a solid center, a thread rubber layer formed by winding thread rubber around the solid center, and a cover enclosing the thread rubber layer. It has been found that by optimizing the diameter, the hardness distribution, the difference between maximum hardness and minimum hardness, the surface hardness, and the deformation under an increasing load of the solid center, the golf ball is given an improved spin performance capable of complying with any of different golf scenes covering a wide range of club and head speed variables. Additionally, the ball affords a good feel, high durability and increased distance.
It has also been found that by further specifying the deformation of the solid center under an increasing load relative to the diameter of the solid center and, if necessary, further specifying the material and hardness of the cover, there is obtained a golf ball in which the solid center not only undergoes a greater, yet not excessive, amount of deformation than conventional solid centers, when hit at a high head speed with a greater impact force as in driver shots, but also undergoes adequate deformation even when hit at a low head speed with a smaller impact force as in short-iron shots. This ensures that the ball is improved in feel, durability and distance. The invention is predicated on these findings.
Accordingly, the invention provides a wound golf ball comprising a solid center, a thread rubber layer formed by winding thread rubber around the solid center, and a cover enclosing the thread rubber layer. The solid center having a middle and an outer surface has a diameter of 29 to 37 mm, has such a hardness distribution that the hardness at the outer surface is lower than the hardness at the middle and the difference between the maximum and the minimum of solid center hardness is at least 6 units on JIS-C hardness scale, has a JIS-C hardness of 50 to 85 at the outer surface, and experiences a deformation of 2 to 4.5 mm when the load applied thereto is increased from an initial load of 1 kg to a final load of 50 kg.
In one preferred embodiment, A representing the diameter (mm) of the solid center and B representing the deformation (mm) of the solid center when the load applied thereto is increased from an initial load of 1 kg to a final load of 50 kg satisfy the following relationship:
xe2x88x920.1xc3x97A+5xe2x89xa6Bxe2x89xa6xe2x88x920.3xc3x97A+14.
In a further preferred embodiment, the cover is composed mainly of an ionomer resin and has a Shore D hardness of 40 to 60.